Molecular cloning of a highly leukemogenic, ecotropic retrovirus from an AKR mouse

Hotspots of MLV integration in the hematopoietic tumor genome

Extensive research has been performed regarding the integration sites of murine leukemia retrovirus (MLV) for the identification of proto-oncogenes. To date, the overlap of mutations within specific oligonucleotides across different tumor genomes has been regarded as a rare event; however, a recent study of MLV integration into the oncogene Zfp521 suggested the existence of a hotspot oligonucleotide for MLV integration. In the current review, we discuss the hotspots of MLV integration into several genes: c-Myc, Stat5a and N-myc, as well as ZFP521, as examined in tumor genomes. From this, MLV integration convergence within specific oligonucleotides is not necessarily a rare event. This short review aims to promote re-consideration of MLV integration within the tumor genome, which involves both well-known and potentially newly identified and novel mechanisms and specifications.

Coupling of receptor interference and a host-dependent post-binding entry deficiency in a gammaretroviral envelope protein

Background

SL3-2 is a unique polytropic murine gammaretroviral isolate that is only able to infect murine cells. We have previously shown that two mutations R212G and T213I located on the surface of the receptor binding domain in a region designated the VR3 loop can alter the species tropism of this envelope protein. This location suggests that the VR3 loop composition has an influence on receptor interaction and thereby affects binding as well as superinfection resistance. In order to investigate this further, we have studied the binding and interference patterns of the SL3-2 envelope and its mutants.

Results

We find unexpectedly that wild type SL3-2 envelope binds equally well to both permissive and non-permissive cells, indicating a post binding defect when interacting with the human Xpr1. Using replication competent viruses containing envelopes from SL3-2 or its mutants we find that the same amino acid mutations can dramatically alter the interference profile of this polytropic ENV, suggesting that the same amino acid changes that cause the post binding defect also influence interaction with the receptor.

Conclusions

The envelope protein of SL3-2 MLV shows an entry defect on non-murine cells. This is coupled to a dramatically reduced ability to interfere with entry of other polytropic viruses. Two point mutations in the VR3 loop of the receptor binding domain of this envelope result both in a much increased interference ability and in removing the post-binding defect on non-murine cells, suggesting that both of these phenotypes are a consequence of insufficient interaction between the envelope and the receptor

The emerging role of microRNAs in immune cell development and differentiation

MicroRNAs (miRNAs) are small RNA strands (20-25 nucleotides) that regulate gene expression by translational repression as well as by messenger RNA degradation. This review will examine the application and function of miRNAs in immune cell development and differentiation.

Control of pathogenicity and disease specificity of a T-lymphomagenic gammaretrovirus by E-box motifs but not by an overlapping glucocorticoid response element

Although transcription factors of the basic helix-loop-helix family have been shown to regulate enhancers of lymphomagenic gammaretroviruses through E-box motifs, the overlap of an E-box motif (Egre) with the glucocorticoid response element (GRE) has obscured their function in vivo. We report here that Egre, but not the GRE, affects disease induction by the murine T-lymphomagenic SL3-3 virus. Mutating all three copies of Egre prolonged the tumor latency period from 60 to 109 days. Further mutating an E-box motif (Ea/s) outside the enhancer prolonged the latency period to 180 days, suggesting that Ea/s works as a backup site for Egre. While wild-type SL3-3 and GRE and Ea/s mutants exclusively induced T-cell lymphomas with wild-type latencies mainly of the CD4(+) CD8(-) phenotype, Egre as well as the Egre and Ea/s mutants induced B-cell lymphomas and myeloid leukemia in addition to T-cell lymphomas. T-cell lymphomas induced by the two Egre mutants had the same phenotype as those induced by wild-type SL3-3, indicating the incomplete disruption of T-cell lymphomagenesis, which is in contrast to previous findings for a Runx site mutant of SL3-3. Mutating the Egre site or Egre and Ea/s triggered several tumor phenotype-associated secondary enhancer changes encompassing neighboring sites, none of which led to the regeneration of an E-box motif. Taken together, our results demonstrate a role for the E-box but not the GRE in T lymphomagenesis by SL3-3, unveil an inherent broader disease specificity of the virus, and strengthen the notion of selection for more potent enhancer variants of mutated viruses during tumor development.

Pvt1-encoded microRNAs in oncogenesis

BACKGROUND

The functional significance of the Pvt1 locus in the oncogenesis of Burkitt's lymphoma and plasmacytomas has remained a puzzle. In these tumors, Pvt1 is the site of reciprocal translocations to immunoglobulin loci. Although the locus encodes a number of alternative transcripts, no protein or regulatory RNA products were found. The recent identification of non-coding microRNAs encoded within the PVT1 region has suggested a regulatory role for this locus.

RESULTS

The mouse Pvt1 locus encodes several microRNAs. In mouse T cell lymphomas induced by retroviral insertions into the locus, the Pvt1 transcripts, and at least one of their microRNA products, mmu-miR-1204 are overexpressed. Whereas up to seven co-mutations can be found in a single tumor, in over 2,000 tumors none had insertions into both the Myc and Pvt1 loci.

CONCLUSION

Judging from the large number of integrations into the Pvt1 locus - more than in the nearby Myc locus - Pvt1 and the microRNAs encoded by it are as important as Myc in T lymphomagenesis, and, presumably, in T cell activation. An analysis of the co-mutations in the lymphomas likely place Pvt1 and Myc into the same pathway.

Retroviral activation of the mir-106a microRNA cistron in T lymphoma

Retroviral insertion into a host genome is a powerful tool not only for the discovery of cancer genes, but also for the discovery of potential oncogenic noncoding RNAs. In a large-scale mouse T lymphocyte tumor screen we found a high density of integrations upstream of the mir-106a microRNA cistron. In tumors containing an integration, the primary transcript encoding the mir-106a cistron was overexpressed five to 20-fold compared with that of control tumors; concomitantly, the mature mir-106a and mir-363 microRNAs were highly overexpressed as well. These findings suggest the mir-106a cistron plays an important role in T cell tumorigenesis.

Tumor model-specific proviral insertional mutagenesis of the Fos/Jdp2/Batf locus

Retroviral activation of the AP-1/ATF super family member Jdp2 was recently reported to be a common event in M-MLV-induced T cell lymphoma in p27-null C57x129 mice as compared to wild type-inoculated mice but has not been found important in other models. On the basis of retroviral tag retrieval from 1190 individual Akv- and SL3-3-induced lymphomas, we here report that insertional mutagenesis into the 250-kb Fos/Jdp2/Batf locus is associated with SL3-3 MLV-induced T but not Akv-induced B cell lymphomas of NMRI and SWR mice. Integration pattern and clonality analyses suggest that Jdp2 participates in SL3-3-induced tumorigenesis distinctly as compared to the M-MLV setting. Northern blot analysis showed Jdp2 to be alternatively spliced in various normal tissues as well as MLV-induced lymphomas. Interestingly, in some tumors, proviral insertion seems to activate different mRNA sub-species. Whereas elevated mRNA levels of the Fos gene could not be correlated with provirus presence, in one case, Northern blot analysis as well as quantitative real-time PCR indicated proviral activation of the AP-1 super family member Batf, a gene not previously reported to be a target of insertional mutagenesis. A novel integration cluster between Jdp2 and Batf apparently did not influence the expression level of either gene, underscoring the importance of addressing expression effects to identify target genes of insertion. Altogether, such distinct insertion patterns point to different mechanism of activation of specific proto-oncogenes and are consequently of importance for the understanding of proviral activation mechanisms as well as the specific role of individual oncogenes in tumor development.

Mutation of all Runx (AML1/core) sites in the enhancer of T-lymphomagenic SL3-3 murine leukemia virus unmasks a significant potential for myeloid leukemia induction and favors enhancer evolution toward induction of other disease patterns

SL3-3 murine leukemia virus is a potent inducer of T-lymphomas in mice. Using inbred NMRI mice, it was previously reported that a mutant of SL3-3 with all enhancer Runx (AML1/core) sites disrupted by 3-bp mutations (SL3-3dm) induces predominantly non-T-cell tumors with severely extended latency (S. Ethelberg, J. Lovmand, J. Schmidt, A. Luz, and F. S. Pedersen, J. Virol. 71:7273-7280, 1997). By use of three-color flow cytometry and molecular and histopathological analyses, we have now performed a detailed phenotypic characterization of SL3-3- and SL3-3dm-induced tumors in this mouse strain. All wild-type induced tumors had clonal T-cell receptor beta rearrangements, and the vast majority were CD3(+) CD4(+) CD8(-) T-lymphomas. Such a consistent phenotypic pattern is unusual for murine leukemia virus-induced T-lymphomas. The mutant virus induced malignancies of four distinct hematopoietic lineages: myeloid, T lymphoid, B lymphoid, and erythroid. The most common disease was myeloid leukemia with maturation. Thus, mutation of all Runx motifs in the enhancer of SL3-3 severely impedes viral T-lymphomagenicity and thereby discloses a considerable and formerly unappreciated potential of this virus for myeloid leukemia induction. Proviral enhancers with complex structural alterations (deletions, insertions, and/or duplications) were found in most SL3-3dm-induced T-lymphoid tumors and immature myeloid leukemias but not in any cases of myeloid leukemia with maturation, mature B-lymphoma, or erythroleukemia. Altogether, our results indicate that the SL3-3dm enhancer in itself promotes induction of myeloid leukemia with maturation but that structural changes may arise in vivo and redirect viral disease specificity to induction of T-lymphoid or immature myeloid leukemias, which typically develop with moderately shorter latencies.

The use of retroviral vectors for gene therapy-what are the risks? A review of retroviral pathogenesis and its relevance to retroviral vector-mediated gene delivery

Retroviral vector-mediated gene transfer has been central to the development of gene therapy. Retroviruses have several distinct advantages over other vectors, especially when permanent gene transfer is the preferred outcome. The most important advantage that retroviral vectors offer is their ability to transform their single stranded RNA genome into a double stranded DNA molecule that stably integrates into the target cell genome. This means that retroviral vectors can be used to permanently modify the host cell nuclear genome. Recently, retroviral vector-mediated gene transfer, as well as the broader gene therapy field, has been re-invigorated with the development of a new class of retroviral vectors which are derived from lentiviruses. These have the unique ability amongst retroviruses of being able to infect non-cycling cells. Vectors derived from lentiviruses have provided a quantum leap in technology and seemingly offer the means to achieve significant levels of gene transfer in vivo.The ability of retroviruses to integrate into the host cell chromosome also raises the possibility of insertional mutagenesis and oncogene activation. Both these phenomena are well known in the interactions of certain types of wild-type retroviruses with their hosts. However, until recently they had not been observed in replication defective retroviral vector-mediated gene transfer, either in animal models or in clinical trials. This has meant the potential disadvantages of retroviral mediated gene therapy have, until recently, been seen as largely, if not entirely, hypothetical. The recent clinical trial of gammac mediated gene therapy for X-linked severe combined immunodeficiency (X-SCID) has proven the potential of retroviral mediated gene transfer for the treatment of inherited metabolic disease. However, it has also illustrated the potential dangers involved, with 2 out of 10 patients developing T cell leukemia as a consequence of the treatment. A considered review of retroviral induced pathogenesis suggests these events were qualitatively, if not quantitatively, predictable. In addition, it is clear that the probability of such events can be greatly reduced by relatively simple vector modifications, such as the use of self-inactivating vectors and vectors derived from non-oncogenic retroviruses. However, these approaches remain to be fully developed and validated. This review also suggests that, in all likelihood, there are no other major retroviral pathogenetic mechanisms that are of general relevance to replication defective retroviral vectors. These are important conclusions as they suggest that, by careful design and engineering of retroviral vectors, we can continue to use this gene transfer technology with confidence.

Disruption of hematopoiesis and thymopoiesis in the early premalignant stages of infection with SL3-3 murine leukemia virus

A time course analysis of SL3-3 murine leukemia virus (SL3) infection in thymus and bone marrow of NIH/Swiss mice was performed to assess changes that occur during the early stages of progression to lymphoma. Virus was detectable in thymocytes, bone marrow, and spleen as early as 1 to 2 weeks postinoculation (p.i.). In bone marrow, virus infection was detected predominantly in immature myeloid or granulocytic cells. Flow cytometry revealed significant reductions of the Ter-119(+) and Mac-1(+) populations, and significant expansions of the Gr-1(+) and CD34(+) populations, between 2 and 4 weeks p.i. Analysis of colony-forming potential confirmed these findings. In the thymus, SL3 replication was associated with significant disruption in thymocyte subpopulation distribution between 4 and 7 weeks p.i. A significant thymic regression was observed just prior to the clonal outgrowth of tumor cells. Proviral long terminal repeats (LTRs) with increasing numbers of enhancer repeats were observed to accumulate exclusively in the thymus during the first 8 weeks p.i. Observations were compared to the early stages of infection with a virtually nonpathogenic SL3 mutant, termed SL3DeltaMyb5, which was shown by real-time PCR to be replication competent. Comparison of SL3 with SL3DeltaMyb5 implicated certain premalignant changes in tumorigenesis, including (i) increased proportions of Gr-1(+) and CD34(+) bone marrow progenitors, (ii) a significant increase in the proportion of CD4(-) CD8(-) thymocytes, (iii) thymic regression prior to tumor outgrowth, and (iv) accumulation of LTR enhancer variants. A model in which disrupted bone marrow hematopoiesis and thymopoiesis contribute to the development of lymphoma in the SL3-infected animal is discussed.

Increased induction of osteopetrosis, but unaltered lymphomagenicity, by murine leukemia virus SL3-3 after mutation of a nuclear factor 1 site in the enhancer

SL3-3 is a murine leukemia virus which is only weakly bone pathogenic but highly T-cell lymphomagenic. A major pathogenic determinant is the transcriptional enhancer comprising several transcription factor binding sites, among which are three identical sites for nuclear factor 1 (NF1). We have investigated the pathogenic properties of NF1 site enhancer mutants of SL3-3. Two different mutants carrying a 3-bp mutation either in all three NF1 sites or in the central site alone were constructed and assayed in inbred NMRI mice. The wild type and both mutants induced lymphomas in all mice, with a mean latency period of 9 weeks. However, there was a considerable difference in osteopetrosis induction. Wild-type SL3-3 induced osteopetrosis in 11% of the mice (2 of 19), and the triple NF1 site mutant induced osteopetrosis in none of the mice (0 of 19), whereas the single NF1 site mutant induced osteopetrosis in 56% (10 of 18) of the mice, as determined by X-ray analysis. A detailed histological examination of the femurs of the mice was carried out and found to support this diagnosis. Thus, the NF1 sites of SL3-3 are major determinants of osteopetrosis induction, without determining lymphomagenesis. This conclusion was further supported by evaluation of the bone pathogenicity of other SL3-3 enhancer variants, the lymphomagenicity of which had been examined previously. This evaluation furthermore strongly indicated that the core sites, a second group of transcription factor binding sites in the viral enhancer, are necessary for the osteopetrosis induction potential of SL3-3.

Replication and pathogenicity of primer binding site mutants of SL3-3 murine leukemia viruses

Retroviral reverse transcription is primed by a cellular tRNA molecule annealed to an 18-bp primer binding site sequence. The sequence of the primer binding site coincides with that of a negatively acting cis element that mediates transcriptional silencing of murine leukemia virus (MLV) in undifferentiated embryonic cells. In this study we test whether SL3-3 MLV can replicate stably using tRNA primers other than the cognate tRNAPro and analyze the effect of altering the primer binding site sequence to match the 3' end of tRNA1Gln, tRNA3Lys, or tRNA1,2Arg in a mouse pathogenicity model. Contrary to findings from cell culture studies of primer binding site-modified human immunodeficiency virus type 1 and avian retroviruses, our findings were that SL3-3 MLV may stably and efficiently replicate with tRNA primers other than tRNAPro. Although lymphoma induction of the SL3-3 Lys3 mutant was significantly delayed relative to that of the wild-type virus, molecular tumor analysis indicated that all the primer binding site-modified viruses induce T-cell lymphomas similar to those induced by the wild-type virus in terms of frequencies of genomic rearrangements within the T-cell receptor beta-chain, the immunoglobulin kappa light chain, and the c-myc locus. Whereas none of the mutants were found to revert to tRNAPro primer utilization, in two tumors resulting from the injection of the SL3-3 Lys3 mutant the primer binding site was altered to match that of a new primer species, tRNA1,2Lys. In addition, recombination with endogenous viruses resulting in the generation of recombinant viruses carrying a glutamine primer binding site was detected in the majority of the tumors induced by the SL3-3 Lys3 mutant as well as in two tumors induced by wild-type SL3-3 and the SL3-3 Arg1,2 mutant.

Increased lymphomagenicity and restored disease specificity of AML1 site (core) mutant SL3-3 murine leukemia virus by a second-site enhancer variant evolved in vivo

SL3-3 is a highly T-lymphomagenic murine retrovirus. The major genetic determinant of disease is the transcriptional enhancer, which consists of a repeated region with densely packed binding sites for several transcription factors, including AML1 (also known as core binding factor and polyoma enhancer-binding protein 2) and nuclear factor 1 (NF1). Previously, we examined the enhancer structure of proviruses from murine tumors induced by SL3-3 with mutated AML1 (core) sites and found a few cases of second-site alterations. These consisted of deletions involving the NF1 sites and alterations in overall number of repeat elements, and they conferred increased enhancer strength in transient transcription assays. We have now tested the pathogenicity of a virus harboring one such second-site variant enhancer in inbred NMRI mice. It induced lymphomas with a 100% incidence and a significantly shorter latency than the AML1 mutant it evolved from. The enhancer structure thus represents the selection for a more tumorigenic virus variant during the pathogenic process. Sequencing of provirus from the induced tumors showed the new enhancer variant to be genetically stable. Also, Southern blotting showed that the tumors induced by the variant were T-cell lymphomas, as were the wild-type-induced lymphomas. In contrast, tumors induced by the original core/AML1 site I-II mutant appeared to be of non-T-cell origin and several proviral genomes with altered enhancer regions could be found in the tumors. Moreover, reporter constructs with the new tumor-derived variant could not be transactivated by AML1 in cotransfection experiments as could the wild type. These results emphasize the importance of both core/AML1 site I and site II for the pathogenic potential of SL3-3 and at the same time show that second-site alterations can form a viral variant with a substantial pathogenic potential although both AML1 sites I and II are nonfunctional.

Second-site proviral enhancer alterations in lymphomas induced by enhancer mutants of SL3-3 murine leukemia virus: negative effect of nuclear factor 1 binding site

SL3-3 is a highly T-lymphomagenic murine retrovirus. Previously, mutation of binding sites in the U3 repeat region for the AML1 transcription factor family (also known as core binding factor [CBF], polyomavirus enhancer binding protein 2 [PEBP2], and SL3-3 enhancer factor 1 [SEF1]) were found to strongly reduce the pathogenicity of SL3-3 (B. Hallberg, J. Schmidt, A. Luz, F. S. Pedersen, and T. Grundström, J. Virol. 65:4177-4181, 1991). We have now examined the few cases in which tumors developed harboring proviruses that besides the AML1 (core) site mutations carried second-site alterations in their U3 repeat structures. In three distinct cases we observed the same type of alteration which involved deletions of regions known to contain binding sites for nuclear factor 1 (NF1) and the addition of extra enhancer repeat elements. In transient-expression experiments in T-lymphoid cells, these new U3 regions acted as stronger enhancers than the U3 regions of the original viruses. This suggests that the altered proviruses represent more-pathogenic variants selected for in the process of tumor formation. To analyze the proviral alterations, we generated a series of different enhancer-promoter reporter constructs. These constructs showed that the additional repeat elements are not critical for enhancer strength, whereas the NF1 sites down-regulate the level of transcription in T-lymphoid cells whether or not the AML1 (core) sites are functional. We therefore also tested SL3-3 viruses with mutated NF1 sites. These viruses have unimpaired pathogenic properties and thereby distinguish SL3-3 from Moloney murine leukemia virus.

Mapping of a major osteomagenic determinant of murine leukemia virus RFB-14 to non-long terminal repeat sequences

Certain isolates of murine leukemia viruses (MuLVs) have, apart from a leukemogenic potential, the capability of inducing diseases of nonhematopoietic tissues in susceptible strains of mice. We have reported on the molecular cloning of a bone-tumorigenic virus, RFB-14 MuLV, which was found to induce benign bone tumors, osteomas, with 100% incidence in mice of the CBA/Ca strain (L. Pedersen, W. Behnisch, J. Schmidt, A. Luz, F. S. Pedersen, V. Erfle, and P. G. Strauss, J. Virol. 66:6186-6190, 1992). In order to analyze the bone tumor-inducing phenotype of RFB-14 MuLV, we have studied the pathogenic potential of recombinant viruses between RFB-14 and the nonosteomagenic, highly leukemogenic SL3-3 MuLV. The recombinants were constructed so as to reveal whether a major determinant of osteomagenicity maps to sequences within or outside the long terminal repeats (LTR). Our data show that a major determinant of the osteoma-inducing potential of RFB-14 MuLV maps to the non-LTR region of the genome. Furthermore, we demonstrate that a strong determinant of leukemogenicity is harbored by the non-LTR region of SL3-3 MuLV.

Sequence tags of provirus integration sites in DNAs of tumors induced by the murine retrovirus SL3-3

The murine retrovirus SL3-3 is a potent inducer of T-cell lymphomas when inoculated into susceptible newborn mice. The proviral integration site sequences were surveyed in tumor DNAs by a simple two-step PCR method. From 20 SL3-3-induced tumors a total of 39 provirus-host junctions were amplified and sequenced. Seven showed homology to known sequences. These included the known common integration site c-myc as well as genes not previously identified as targets of provirus integration, namely N-ras and the genes coding for major histocompatibility complex class 11 E-beta, protein kinase C-eta, and T-cell receptor beta-chain. Among these genes, the integrations in c-myc as well as the one in N-ras were found to be clonal. One of the remaining 32 proviral integration site sequences that show no similarities to known sequences may represent a common integration site, as 2 of the 20 tumors demonstrated clonal provirus insertion into this region.

Akv murine leukemia virus enhances lymphomagenesis in myc-kappa transgenic and in wild-type mice

The contribution of endogenous retroviruses to the multistep process of lymphomagenesis was investigated in wild-type mice and in two different myc-kappa transgenic mouse lines by infection with Akv. This retrovirus is derived from the endogenous ecotropic provirus of the AKR mouse and was previously considered to be nonlymphomagenic. The mice of the two myc-k transgenic lines are predisposed to B-cell lymphomagenesis and were therefore considered to be more susceptible to Akv. For comparison, the same mouse strains were also infected with the exogenous Moloney murine leukemia virus (MoMuLV). Both MoMuLV and Akv increased the tumor incidence and shortened the tumor latency period in wild-type mice and in the transgenic mouse lines. The differences in pathogenicity, number of provirus integrations, and level of virus expression between MoMuLV and Akv indicate different mechanisms of lymphomagenesis: while MoMuLV induced tumors apparently by insertional mutagenesis involving common integration sites similar to previous reports, the enhancement of lymphomagenesis by Akv seems to be directed by other mechanisms.

Akv murine leukemia virus enhances bone tumorigenesis in hMT-c-fos-LTR transgenic mice

hMt-c-fos-LTR transgenic mice (U. Rüther, D. Komitowski, F. R. Schubert, and E. F. Wagner. Oncogene 4, 861-865, 1989) developed bone sarcomas in 20% (3/15) of females at 448 +/- 25 days and in 8% (1/12) of males at 523 days. After infection of newborns with Akv, an infectious retrovirus derived from the ecotropic provirus of the AKR mouse, 69% (20/28) of female animals and 83% (24/29) of males developed malignant fibrous-osseous tumors. The tumors in infected transgenics developed with higher frequency and a 200-days shorter mean tumor latency period. The hMt-c-fos-LTR transgene was expressed in all the fibrous-osseous tumors. They also showed newly integrated Akv proviruses, but in most tumors Akv was detected and expressed in only a small number of the tumor cells. Wild-type C3H mice infected with Akv developed benign osteomas with an incidence of 33% and a latency period of 474 days. The data indicate that Akv exerts distinct pathogenic effects on the skeleton. In hMt-c-fos-LTR transgenic mice, predisposed to bone sarcomagenesis, Akv acts synergistically with the fos transgene, resulting in the development of fibrous-osseous tumors.

Basic helix-loop-helix proteins in murine type C retrovirus transcriptional regulation

E boxes, recognition sequences for basic helix-loop-helix (bHLH) transcription factors, are detected in the enhancer and promoter regions of several murine type C retroviruses. Here we show that ALF1, a member of bHLH protein family of transcription factors, in vitro binds with differing affinities to distinct E-box sequences found in the U3 regulatory regions of Friend, Moloney, SL3-3, and Akv murine leukemia viruses (MLVs) as well as Friend spleen focus-forming virus (SFFV). In NIH 3T3 fibroblasts, ALF1 overexpression elevated transcription from the U3 region of Moloney MLV and the complete long terminal repeat regions of Friend SFFV, Akv MLV, and SL3-3 MLV but neither from the U3 region nor from the complete long terminal repeat of Friend MLV. Introduction of mutations in the Akv MLV E boxes showed the E-box cis elements to be required for the function of ALF1 as a transcription factor. ALF1 and the glucocorticoid receptor, with overlapping DNA binding sequences, did not act synergistically with respect to transcriptional trans activation of expression from the Akv MLV promoter-enhancer region. We conclude that ALF1 in vivo may be an important transcription regulator for Akv, SL3-3, and Moloney MLVs as well as for Friend SFFV.

Efficient replication and expression of murine leukemia virus with major deletions in the enhancer region of U3

The effect of deletions within the enhancer region in the U3 part of the LTR derived from the murine retrovirus Akv was studied. The deletions were stably transmitted through normal virus replication as shown by sequence analysis of cloned polymerase chain reaction product of the cDNA copy of the viral RNA. Genetic tagging of the retrovirus with lacO facilitated the analysis. Among the individual mutated LTRs an over 100-fold difference in a transient expression assay was previously detected. This difference was not revealed in studies of viral replication in cell culture, where the expression level of virus with the deleted LTRs all reached the level of virus with the intact LTR. We propose that stimulatory cis-acting sequences either adjacent to the site of proviral integration or in the coding regions of the provirus may compensate for deletions in the LTR.

Tagging the genome of the murine leukemia retrovirus SL3-3 by a bacterial lac operator sequence

The bacterial lactose operator (lacO) was introduced into the PstI site of the long terminal repeat of the SL3-3 murine leukemia virus, generating a virus, SL3-3lacO, that can replicate in NIH3T3 cell cultures. DNA sequences harboring the lacO sequence might be recovered by molecular cloning in Escherichia coli lac+ lacZ+ using bacteriophage lambda or plasmid vectors. The high copy numbers of the lacO sequence titrate out the lac repressor, leading to the induction of the lac operon in the host. We show here that the lacO and the proviral sequences are carried stably together in the genomes of SL3-3lacO-infected cell cultures and in viral particles. This system is designed to facilitate studies on the provirus and the site of viral integration.

A host gene regulates the structure of the transmembrane envelope protein of murine leukemia viruses

Heterogeneity in the structure of the envelope proteins has been observed in many human and animal retroviruses and may influence pathogenicity. However, the biological significance of this heterogeneity and the mechanisms by which it is generated are poorly understood. We have studied a mouse model in which the envelope gene structure of lymphoma-associated viruses appears to be controlled by a single host gene. The inoculation of HRS and CWD mice with a leukemogenic murine leukemia virus (MuLV) results in recombination between the injected virus and envelope gene sequences of endogenous retroviruses. The genomes of HRS (class I) env recombinants and CWD (class II) env recombinants differ in the sequences encoding the NH2-terminal portion of the transmembrane envelope protein (TM). We have shown that an HRS gene linked to the MHC on chromosome 17 mediates a dominant selection for recombinant retroviruses with the class I envelope gene structure. CBA mice, which share the H-2k haplotype with HRS, also carry the dominant allele at this locus. This system provides a useful model for studies of host factors involved in the selection of specific variants of pathogenic retroviruses.

Differential protein binding in lymphocytes to a sequence in the enhancer of the mouse retrovirus SL3-3

An electrophoretic mobility shift assay was used to characterize interactions of nuclear proteins with a DNA segment in the enhancer element of the leukemogenic murine retrovirus SL3-3. Mutation of a DNA sequence of the 5'-TGTGG-3' type decreased transcription in vivo specifically in T-lymphocyte cell lines. Extracts of nuclei from different T-lymphocyte cell lines or cells from lymphoid organs resulted in much higher amounts of complexes in vitro with this DNA sequence than did extracts from other cell lines or organs tested. Differences were also found in the sets of complexes obtained with extracts from the different types of cells. The DNA sequence specificities of the different SL3-3 enhancer factor 1 (SEF1) protein complexes were found to be distinct from those of several other previously identified DNA motifs of the TGTGG type because of differences in several nucleotides critical for binding and because these other DNA motifs could not compete with the identified DNA sequence for binding of SEF1. Limited treatment with several different proteases cleaved the SEF1 proteins such that their DNA-binding domain(s) remained and created complexes with decreased and nondistinguishable electrophoretic mobility shifts and with new properties. These results indicate that the SEF1 proteins have a structure with a flexible and relatively vulnerable hinge region linking a DNA-binding domain(s) to a more variable domain(s) with other functions. We suggest that the binding of SEF1 is an essential factor for the T-cell tropism of SL3-3 and the ability of this virus to cause T-cell lymphomas.

Tissue specific sequence motifs in the enhancer of the leukaemogenic mouse retrovirus SL3-3

The long terminal repeat (LTR) of the retrovirus SL3-3 determines its tropism for T-lymphocytes and its ability to induce T-cell lymphomas in mice. We have studied the ability of different DNA sequences located upstream of the "TATA" box in the LTR of SL3-3 to enhance transcription in T-lymphocyte cell lines and other cell lines, employing a transient assay and quantitative S1 nuclease mapping. The enhancer was found to be composed of many DNA domains which determines different activities in different cell lines. We find enhancer sequence motifs with a high T-lymphocyte specificity in the DNA repetitions of the LTR, and other enhancer motifs active in a broader range of cells in the surrounding DNA segments. The localization of sequences preferentially active in T-cells within the repeated sequences containing differences between SL3-3 and the very closely related Akv virus, which is without the T cell tropism and leukaemogenicity of SL3-3, supports the notion that the enhancer sequence motifs with T-cell preferences are primary determinants of these properties.

Mechanism of selection of class II recombinant murine leukemia viruses in the highly leukemic strain CWD

The development of spontaneous lymphomas in CWD mice is associated with the expression of endogenous ecotropic murine leukemia viruses (MuLV) and the formation of recombinant viruses. However, the pattern of substitution of nonecotropic sequences within the envelope genes of the CWD class II recombinant viruses differs from that seen in class I recombinant MuLVs of AKR, C58, and HRS mice. To determine how CWD host genes might influence the envelope gene structure of the recombinant viruses, we characterized the responses of these mice to two different types of exogenous MuLVs. Neonatal mice injected the HRS class I recombinant PTV-1 became infected and developed T-cell lymphomas more rapidly than controls did. The inoculation of CWD mice with the leukemogenic AKR ecotropic virus SL3-3 led to the formation of recombinant MuLVs with a novel genetic structure and class II-like envelope genes, although SL3-3 generates class I recombinants in other strains. These results suggest that the absence of class I recombinant MuLVs in CWD mice is not related to the restriction of the replication or oncogenicity of class I viruses or to the absence of an appropriate ecotropic virus that can generate class I recombinants. More likely, the genes of CWD mice that direct the formation or selection of class II recombinant viruses affect the process of recombination between the ecotropic and nonecotropic envelope gene sequences.

Differential protein binding in lymphocytes to a sequence in the enhancer of the mouse retrovirus SL3-3

An electrophoretic mobility shift assay was used to characterize interactions of nuclear proteins with a DNA segment in the enhancer element of the leukemogenic murine retrovirus SL3-3. Mutation of a DNA sequence of the 5'-TGTGG-3' type decreased transcription in vivo specifically in T-lymphocyte cell lines. Extracts of nuclei from different T-lymphocyte cell lines or cells from lymphoid organs resulted in much higher amounts of complexes in vitro with this DNA sequence than did extracts from other cell lines or organs tested. Differences were also found in the sets of complexes obtained with extracts from the different types of cells. The DNA sequence specificities of the different SL3-3 enhancer factor 1 (SEF1) protein complexes were found to be distinct from those of several other previously identified DNA motifs of the TGTGG type because of differences in several nucleotides critical for binding and because these other DNA motifs could not compete with the identified DNA sequence for binding of SEF1. Limited treatment with several different proteases cleaved the SEF1 proteins such that their DNA-binding domain(s) remained and created complexes with decreased and nondistinguishable electrophoretic mobility shifts and with new properties. These results indicate that the SEF1 proteins have a structure with a flexible and relatively vulnerable hinge region linking a DNA-binding domain(s) to a more variable domain(s) with other functions. We suggest that the binding of SEF1 is an essential factor for the T-cell tropism of SL3-3 and the ability of this virus to cause T-cell lymphomas.

An MuLV transmission vector system designed to permit recovery in E. coli of proviral and cellular flanking sequences

We have introduced a bacterial suppressor gene (supF) into the long terminal repeat of a molecular clone of the murine leukemia virus (MuLV) SL3-3. A panel of replication competent virus was derived that replicates to high titers in NIH3T3 cells in culture. The tRNA gene is stably carried in the provirus. The supF and viral sequences are present in equimolar amounts in the RNA genome of the expressed recombinant virus. The proviral sequences containing supF can be recovered by cloning into a lambda vector carrying amber mutations. The DNA sequences in the recovered lambda recombinants show a high degree of stability. The presented system should facilitate the study of the interaction between proviral and cellular sequences flanking the integration site.

AKR ecotropic murine leukemia virus SL3-3 forms envelope gene recombinants in vivo

The ecotropic AKR virus SL3-3 was injected into neonatal mice of the high-leukemia strains HRS/J and CWD/J and the low-leukemia strains CBA/J, SEA/J, and NIH Swiss. SL3-3 was highly leukemogenic in each strain, and 90% of the inoculated animals died by 6 months of age. T1 oligonucleotide fingerprint analysis of the genomic RNAs of viruses recovered from 9 of 13 leukemic animals revealed the presence of the SL3-3 virus and recombinant viruses with polytropic virus-related envelope gene sequences. Recombinant proviruses were detected by the Southern blot technique in the DNAs of 17 of 18 tumors. The pattern of substitutions within the envelope genes of the SL3-3 recombinant viruses appeared to be dependent on the strain of the animal. These observations indicate that the SL3-3 virus formed envelope gene recombinants in vivo in each of the strains that were studied. However, the role of these recombinants during leukemogenesis remains to be defined.

Oncogenic retrovirus from spontaneous murine osteomas. II. Molecular cloning and genomic characterization

An N-ecotropic murine leukemia virus (OA MuLV), originally isolated from spontaneous osteomas of strain 101 mice, was molecularly cloned. The virus induces osteomas, osteopetrosis, and malignant lymphomas in NMRI mice. The cloned virus was analyzed by heteroduplex analysis, restriction enzyme mapping, and oligonucleotide mapping. The data show a very close relationship to the endogenous Akv prototype virus with some differences in the gag and the env region. The nucleotide sequence of the U3 region of OA MuLV LTR revealed a structure within the presumable enhancer region very similar to the U3 sequences of the FBJ murine sarcoma virus and its associated helper virus. The significance of these specific structures for the oncogenicity of the virus and the development of the typical disease pattern is discussed.

Ecotropic and mink cell focus-forming murine leukemia viruses integrate in mouse T, B, and non-T/non-B cell lymphoma DNA

Structures of somatically acquired murine leukemia virus (MuLV) genomes present in the DNA of a large panel of MuLV-induced C57BL and BALB/c B and non-T/non-B cell lymphomas were compared with those present in MuLV-induced T-cell lymphomas induced in the same low-"spontaneous"-lymphoma-incidence mice. Analyses were performed with probes specific for the gp70, p15E, and U3-long terminal repeat (LTR) regions of ecotropic AKV MuLV and a mink cell focus-forming virus (MCF)-LTR probe annealing with U3-LTR sequences of a unique endogenous xenotropic MuLV, which also hybridizes with U3-LTR sequences of a substantial portion of somatically acquired MCF genomes in spontaneous AKR thymomas. The DNAs of both T- and B-cell tumors induced by neonatal inoculation with the highly oncogenic C57BL-derived MCF 1233 virus predominantly contain integrated MCF proviruses. In contrast, the DNAs of more slowly developing B and non-T/non-B cell lymphomas induced by poorly oncogenic ecotropic or MCF C57BL MuLV isolates mostly contain somatically acquired ecotropic MuLV genomes. Approximately 50% of the spontaneous C57BL lymphoma DNAs contain somatically acquired MuLV genomes. None of the integrated MuLV proviruses annealed with the MCF-LTR probe, which indicates a clear difference in LTR structure with a substantial portion of the somatically acquired MuLV genomes present in the DNA of spontaneous AKR thymomas. This study stresses a dominant role of MuLV with ecotropic gp70 and LTR sequences in the development of slowly arising MuLV-induced B and non-T/non-B cell lymphomas.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Suppression of retroviral propagation and disease by suramin in murine systems

Retroviral propagation crucially depends on reverse transcriptase (RT). We have developed murine models to test the biological effectiveness of the RT inhibitor suramin. The drug was active in our assay system, which includes (i) inhibition of RT activity in the murine T-cell tropic virus SL3-3 and Rauscher murine leukemia virus (MuLV), (ii) inhibition of plaque formation in the XC plaque assay, (iii) inhibition of viral infection of cultured murine T cells, and (iv) inhibition of splenomegaly induced by Rauscher MuLV in BALB/c mice. Suramin decreases viral titers significantly, even if started 36 hr after infection. Viral titers and number of infected cells increased to control levels after removal of the drug. BALB/c mice treated i.v. with 40 mg of suramin per kg twice per week following infection with Rauscher MuLV showed a 35% decrease in splenomegaly. Suramin is an active antiretroviral agent whose effect on retroviral propagation is reversible. We conclude that it acts as a virustatic drug and that long-term administration of suramin will be necessary if it is used for experimental treatment of human retroviral illnesses such as the acquired immune deficiency syndrome.

Tissue-specific transcription preference as a determinant of cell tropism and leukaemogenic potential of murine retroviruses

Inoculation of susceptible strains of mice with the SL3-3 strain of murine leukaemia viruses induces T-cell lymphomas, whereas injection of the Akv strain does not. Recombinant viruses that contain the long terminal repeat (LTR) of the SL3-3 virus and the gag, pol and env genes of the Akv virus are also leukaemogenic. The cell-type specificity of leukaemias induced by viruses containing different LTR sequences is due in part to the ability of the virus to replicate in the appropriate cellular environment. One explanation of the role of the LTR in determination of both cell tropism and leukaemogenic potential is that the LTR encodes tissue-permissive transcriptional elements. We report here that there are differences in the transcriptional activity of the SL3-3 and Akv LTR sequences in different murine cell types, and that the sequences present in the LTR of SL3-3 exhibit significantly enhanced transcriptional activity in T cells compared with the corresponding region of the Akv LTR. The results suggest that transcriptional elements are primary determinants of cell tropism and of leukaemogenicity of these viruses.

Differences in lymphomagenic properties of AKR mouse retroviruses

Long-term studies on lymphomagenicity of several AKR mouse retroviruses have shown that the biologically cloned ecotropic SL3-3c virus is the most lymphomagenic of all viruses tested. This fact was demonstrated by lymphomagenicity in five mouse strains SJL, C3Hf/Bi, C3H/HeJ, CBA/H, and NFS, and lymphoma acceleration in AKR mice. The incidence was higher and latent periods shorter than that found with the other retroviruses tested (SL3-1c, SL3-2c, MCFc, and GMuLVc). In addition, it was the only retrovirus found to be highly oncogenic in the C3H/HeJ and CBA/H strains. Lack of lymphomagenicity of MCFc in CBA/H strain was shown to be due to a block in viral replication. Addition of nononcogenic Akv ecotropic virus did not affect this lack of oncogenicity. The lymphomas developing in CBA/H and SJL mice after neonatal inoculation of SL3-3c virus only produced lymphomagenic ecotropic virus. Thus, SL3-3c lymphomagenesis is most likely due solely to the action of that virus. These studies indicate that pure ecotropic AKR viruses can be highly leukemogenic.

Determination of the leukaemogenicity of a murine retrovirus by sequences within the long terminal repeat

Although the murine retrovirus SL3-3 is highly leukaemogenic, in both the structure of its genome and in its properties of replication in tissue culture it closely resembles the nonleukaemogenic retrovirus Akv (refs 3, 4). An earlier investigation of the properties of recombinant SL3-3-Akv viruses localized the major determinant of leukaemogenicity outside the env gene, in a region of the viral genome that includes the gag gene and the noncoding long terminal repeat (LTR). To localize the determinant of SL3-3's leukaemogenicity more precisely we have now construced a recombinant provirus containing the LTR of SL3-3 and the coding region of Akv. The leukaemogenicity of these recombinants demonstrates that the determinant of leukaemogenicity lies within the SL3-3 LTR. Nucleotide sequencing of the LTRs of SL3-3 and Akv shows that they differ by a set of changes in the region thought to contain a transcriptional enhancer element. We suggest that enhancer region sequences are the major determinants of leukaemogenicity in these viruses.

Genes affecting mink cell focus-inducing (MCF) murine leukemia virus infection and spontaneous lymphoma in AKR F1 hybrids

An assessment of the importance of mink cell focus-inducing (MCF)-type recombinant murine leukemia viruses in spontaneous thymic lymphomagenesis and of the genetic factors affecting its occurrence was carried out with F1 hybrids between AKR and various other inbred strains. There was generally close agreement between the frequency of detection of MCF virus, of thymocyte antigenic amplification in the preleukemic period, and of spontaneous lymphoma. Also, hybrid combinations with moderate to high spontaneous lymphoma were uniformly susceptible to lymphoma induction by neonatal inoculation of MCF 247 virus, while lower leukemic hybrids were at least partially resistant to the induced disease. At least four resistance genes can be identified as affecting the disease in the various hybrids: Fv-1, Rmcf, an unidentified gene carried by the C57 series of mice and SJL, and an unidentified minor gene carried by several other strains.

Localization of the leukemogenic determinants of SL3-3, an ecotropic, XC-positive murine leukemia virus of AKR mouse origin

SL3-3 is a potent leukemogenic retrovirus that closely resembles the non-leukemogenic virus Akv. Both viruses were isolated from AKR mice, have ecotropic host ranges, and form plaques in the XC assay. They differ at only 1 to 2% of the nucleotides in the viral genomes but differ markedly in virulence properties. SL3-3 induces leukemia in a high percentage of inoculated AKR, C3H, CBA, and NFS mice, whereas Akv does not induce disease in any of these strains. To determine which region of the genome accounts for the leukemogenic potential of SL3-3, we constructed recombinant genomes between molecular clones of SL3-3 and Akv. Recombinant, viral DNA genomes were cloned and then were transfected onto NIH 3T3 fibroblasts to generate infectious virus. The recombinant viruses were tested for leukemogenicity in AKR/J, CBA/J, and C3Hf/Bi mice. We localized the primary leukemogenic determinant to a 3.8-kilobase fragment of the SL3-3 genome containing the viral long terminal repeat, 5' untranslated sequences, gag gene, and 5', 30% of the pol gene. Reciprocal recombinants containing the equivalent region from Akv, linked to the env gene and the remainder of the pol gene from SL3-3, did not induce leukemia. We conclude that the primary virulence determinant of SL3-3 lies outside the region of the genome that encodes the envelope proteins gp70 and p15E.